A low resistance means that the electrons can move easily, therefore the current will be high.

If the metal is heated, the metal ions will vibrate more. This means that there is a greater chance of an electron bumping into a metal ion and so the flow of electrons is reduced. i.e. the resistance is increased.

Explaining the shape of the current-voltage graph for the bulb

Curr

ent

(A)

Voltage (V)

Cold metal = low resistance Hot metal = high resistance

The diode is a non-ohmic conductor. It allows the current to flow in one direction only.

The Diode

Voltage (V)

Cu

rren

t (A

)diode

Allows the current to flow in the forward direction

Doesn’t allow the current to flow

(has infinite resistance in the reverse direction)

Plotting a current-voltage graph for a diode produces this shape line

A

B

C

Circuit 1

which bulb will light up (notice the position of the diodes).

A

B

C

answer

Circuit 1

A & B only

A

B

C

Circuit 2

which bulb will light up (notice the position of the diodes).

A

B

C

answer

Circuit 2

None of them

A

B

C

Circuit 3

which bulb will light up (notice the position of the diodes).

A

B

C

answer

Circuit 3

A & C only

LDRs and ThermistorsLDRs and Thermistors

Method:

• Connect the LDR to a multimeter and set it to 20kΩ,

• Place a desk lamp so that the bulb is 10cm away from the LDR and record the resistance

• Vary the desk lamp distance and measure the resistance accordingly.

• Light intensity can be expressed as an ordered variable: bright, medium and dim.

• Plot a graph of resistance against light intensity.

LDR investigation

While carrying out the experiment:

Explain how you are keeping your test fair

Explain how you are making your readings accurate

Draw a diagram of the experiment

You will carry out an investigation to measure the resistance of an LDR at different light intensities.

An LDR is an electric component whose resistance varies with light intensity.

Method:

1. Heat 250ml of water using a bunsen burner to 80ºC then turn the bunsen burner off,

2. Immerse the thermistor in the hot water,

3. Connect the thermistor to a multimeter and set it to 20kΩ,

4. Record the temperature of the water as it cools down

5. Record the resistance of the thermistor for each corresponding temperature

6. Plot a graph of resistance against temperature.

Thermistor investigation

You will carry out an investigation to measure the resistance of a thermistor at different temperatures.

A thermistor is an electric component whose resistance varies with temperature.

While carrying out the experiment:

Explain how you are keeping your test fair

Explain how you are making your readings accurate

Draw a diagram of the experiment

Resi

stan

ce

temperature

Voltage (V)

Curr

ent

(A)

high temperature

low temperature

Thermistor

Thermistor – resistance DECREASES when temperature INCREASES

Resi

stan

ce

Light intensity Voltage (V)

Curr

ent

(A)

light

dark

LDR

Light dependant resistor – resistance DECREASES when light intensity INCREASES

Light Dependent Resistor/ LDR: Its resistance decreases as the light intensity increases. (The higher the illumination, the higher the current

Temperature Dependent Resistor /Thermistor: Its resistance decreases as the temperature increases. (The higher the temperature, the higher the current).

Thermistors and LDRs can work as Sensors: A sensor is a component that allows a physical quantity to change its resistance and hence control the amount of current flowing in a circuit.

A

In this circuit, the LDR can work as a lightmeter. The higher the ammeter reading, the higher the illumination.

A

In this circuit, the thermistor can work as a thermometer. The higher the ammeter reading, the higher the temperature.